Seems to me to buy a cheap primer gun w a cup under the gun.
Still won't be able to shoot straiight up, but w turning the pickup tube toward the back adding a short rubber ****** to it, and turning vent hole tube up front, you shopuld be able to shoot closer to plumb.
Probably fill cup less than half full.
Marc
To get around wll the nooks and such, you almost need a pressure pot, which will allow you to point the sprayhead in any orientation without either bumping something or loss of paint. Keep in mind, that as you spray upwards, a good amount of paint will snowfall into your face. Laying on your back is the least favorable position. Lift, better. Rotisserie, most excellent choice. Good luck.
I've been out of the body shop for 8 years. I think it was 3m that had a system that used a plastic cup with a liner that you put the paint into. You then screwed it onto you paint gun. If you held your gun upside down and pulled the trigger it would pull all the air out and you could paint holding the paint gun in any direction.